This LEGO pendulum clock is truly practical to use as a clock: it runs for over 72 hours or 3 days, it is very easy to rewind in just a few seconds, and it can be accurate to within a minute per day. A screw on the pendulum bob allows precise adjustment of the effective length of the pendulum. The hands can be easily moved to set the time. These features all contribute to making it possible to have a real working clock made of LEGO requiring only a few seconds of rewinding every 3 days!
LEGO pendulum clock
This video shows how the clock works, and shows what some of the parts looked like after running continuously for about one year.
This clock operates similarly to most mechanical pendulum clocks:
- A drive weight slowly falls, providing energy to the mechanism through a series of gears to an escapement.
- The escapement provides a small amount of energy to a pendulum to keep it swinging, and the pendulum regulates how quickly the escapement wheel can turn.
- Gears connected to the escapement drive the hands of the clock such that if the pendulum maintains a particular period of oscillation, the hands will keep time correctly.
The escapement is a Galileo escapement built around a 40 tooth gear as the escapement wheel.
Closeups of the escapement
The knob on the pendulum bob allows the effective length of the pendulum to be precisely adjusted, enabling very accurate time-keeping.
The clock is driven by a 560 gram weight hanging on a chain, which falls 1.9 cm/hour, for a power consumption of about 29 microwatts. To rewind, simply lift the drive weight. The chain drives a freewheel, which allows the chain to be pulled freely through the clock in reverse. The freewheel rewinding mechanism has minimal friction while the drive is engaged, unlike my previous designs that used a differential with a one-way ratchet. The freewheel is also more reliable than my ratchet designs.
Pendulum bob and drive freewheel
The chain is a loop connected back to the bottom of the weight, to maintain a consistent drive force by balancing the chain.
Here is an Stud.io file and building instructions for this clock. There are some differences not shown in the Stud.io file or instructions:
- The pendulum in the video is made with steel axles from the Brick Machine Shop, in an attempt to make the pendulum less sensitive to temperature. However, these appear to be no longer available, and so the pendulum will require a different length.
- The drive weight uses large washers, which are not part of the Stud.io file. You’ll have to find similar washers or get creative to design your own weight! I took a technic beam to the hardware store and bought the biggest washers with the tightest fit I could find.
- The length of the chain can vary, but to run for 3 days, you’ll need about 200 of the tread links. Of course, if you want to use more chain, the clock could probably run for 4 days or even longer if mounted high enough.
- I couldn’t figure out how to connect the hoses in the clock face, so these parts are detached from the model. They might need to be cut to length.
If you do attempt to build the clock, here are some additional tips and comments:
- The escapement will need quite a bit of effort to tune. The goal is to minimize free rotation of the escapement wheel (which is wasted energy) while allowing the escapement to operate.
- Make sure all of the axles spin very freely, any extra friction will make the clock not run correctly!
- It helps to mount the clock on a beam that can be rotated to adjust the escapement.
- The “feet” of this clock are designed to hang on a mount similar to a “French cleat”.
- Just like a real clock, it is best to set the time by moving the hands clockwise. I’ve never noticed disruption to the escapement tuning to move the hands counter-clockwise, but it might be possible.
- I removed some of the friction bumps from the friction peg/axle part in step 20, to reduce the friction when moving the hands to set the time. I left 2 of the 8 bumps, on opposite sides of the peg.
- If you run it continuously, the escapement pallets will wear out in 6 to 12 months, and will need to be replaced. You could lubricate these parts to reduce the wear… But they are easy to replace and enough of these parts to last 100 years can be bought on bricklink for under $5.
If you want to see more LEGO clocks, check out David Ziemkiewicz, Ben Van de Waal and KEvronista on YouTube. The idea for a knife edge pendulum suspension comes from their videos, and they have a lot more cool clock videos to see.